The second link is the GT version, whereas the first one is the vanilla version (standard 6600)
What do you want to be playing?
The 6600GT is rather old now, and won't play new games very well. The 6600 you'll find has ben discontiuned, and so you can't buy it from there anyway.
The third link won't work
What card was that?
The Nvidia series goes thus:
The GTX being the best version of the card